1. Field of the Invention
The present invention relates generally to vehicles and electromagnetic clutches for compressors for use in air conditioning systems such vehicles. In particular, the present invention relates to vehicles and electromagnetic clutches for compressors, in which an excessive temperature elevation in an electromagnetic clutch may be accurately detected, thereby ensuring proper operation of the electromagnetic clutch and a compressor.
2. Description of Related Art
Generally, in the drive control of a compressor used in a refrigeration cycle of an air conditioning system for vehicles, the transmission and interruption of a drive force from a drive source, e.g., an engine, to the compressor are controlled by an electromagnetic clutch. FIG. 5 depicts a schematic diagram of a known electromagnetic clutch for a compressor. In FIG. 5, electromagnetic clutch 101 is mounted on compressor 100 for controlling the transmission of a drive force from a drive source, such as an engine of a vehicle. Electromagnetic clutch 101 has electromagnetic coil 102, and electromagnetic coil 102 is connected to a power source 104 via air conditioning relay 103 and temperature switch 106. Further, electromagnetic clutch 101 has armature 108 rotated integrally with a drive shaft (not shown) of compressor 100 and rotor 109, to which a driving force from a drive source (e.g., an engine) is applied through a drive force transmission means, such as a pulley (not shown). When electromagnetic coil 102 is excited by closing the contact point of relay 103 and thereby causing a current to flow in electromagnetic coil 102 in response to a signal from air conditioning control unit 107, armature 108 is attracted toward rotor 109 by the electromagnetic attractive force generated by electromagnetic coil 102, and armature 108 and rotor 109 are brought into contact with each other. Consequently, the driving force is transmitted to the drive shaft of compressor 100, thereby driving compressor 100. In contrast, when the excitation of electromagnetic coil 102 is terminated by opening the contact point of relay 103, armature 108 is separated from rotor 109, thereby interrupting the transmission of the driving force.
In addition, temperature fuse 105 is interposed between electromagnetic coil 102 and temperature switch 106, as shown in Japanese Utility Model Laid-Open Nos. 55-175635 and JP-A-2000-230579. When the drive shaft of compressor 100 becomes locked for certain reasons, slippage occurs between armature 108 and rotor 109 and the temperature of a contact portion thereof is elevated to an excessive level by friction. Consequently, temperature fuse 105 melts, and the circuit is opened. As a result of this fuse failure, driving of compressor 100 is stopped, overload conditions of electromagnetic clutch 101 and compressor 100 are prevented, and further, an overload condition to the drive source is prevented, thereby preventing these devices from being damaged.
Temperature switch 106 also is provided on a surface of compressor 100 for detecting the temperature of the discharged heat transfer medium, e.g., the refrigerant. When the drive shaft of compressor 100 becomes locked for certain reasons and the temperature of the heat-transfer medium discharged from compressor 100 increases to an excessive level, temperature switch 106 operates to open, the flow of current to electromagnetic coil 102 is interrupted, armature 108 is separated from rotor 109, the transmission of the driving force from a drive source is interrupted, and electromagnetic clutch 101 and compressor 100 and, ultimately, the drive source, may be protected.
In the above-described circuit, however, because electromagnetic coil 102 and temperature fuse 105 are connected in series, even if temperature fuse 105 melts, control unit 107 cannot detect the failure of temperature fuse 105 unless another detection means is provided. Therefore, even in a situation in which temperature fuse 105 melts, control unit 107 may continue to operate to direct a flow of current to electromagnetic coil 102. Further, when temperature switch 106 operates, a similar phenomenon may occur, specifically, in the above-described circuit, air conditioning control unit 107 is unable to recognize an abnormal condition of compressor 100, unless another detection means is provided. Consequently, because control unit 107 continues to operate heat exchangers and the like even when compressor 100 is stopped, power may be wasted, and operation of the vehicle may be adversely affected. Moreover, if compressor 100 is a hybrid compressor driven by two drive sources different from each other, e.g., an engine and an electric motor incorporated into the compressor, a malfunction may occur, such that the compressor may be attempted to be driven by the incorporated electric motor, even during an abnormal conditions.